1. Field of the Invention
This invention relates to a subscription television system, and more particularly, to a system in which television signals are scrambled for broadcasting, with the scrambled signals being transmitted in a form suitable for recording by means of a recording and playback apparatus such as a video tape recorder.
2. Description of the Prior Art
Recently, a subscription television system has been proposed in which a specific program is transmitted by a broadcasting station after being scrambled. The transmitted program can be received only by the subscribers having special decoders which are supplied based on a contract with the broadcasting station. A small scale experimental broadcast according to such a system is being tried in one area. In the scrambled broadcast, the television signals comprising the specific program are scrambled by changing the level thereof over an optional period or by replacing the order of the signals and then broadcasting the changed signals from the broadcasting station. Accordingly, even if the scrambled television signals are received by an ordinary or conventional receiver, correct pictures and sound can not be reproduced. The normal pictures and sound can be received only by the subscribers who have a contract with the broadcasting station and accordingly have a decoder which can descramble the signals scrambled at the station.
In such a system, the following conditions must be met;
(1) A simultaneous descrambling can not be made while the program is being received in the scrambled broadcasting. PA1 In short, the reception of a normal unscrambled program is not possible in real time. PA1 (2) The program in the scrambled broadcasting must be recorded once in a video tape recorder (VTR) before reproduction. PA1 (3) For the tape in which the program of the scrambled broadcast is recorded, a VTR with a decoder must be used to reproduce the program correctly. PA1 (4) Any type of VTR can be used. PA1 (5) The modification to a conventional VTR, i.e., the addition of a decoder, must minimized. PA1 (6) The scrambled broadcast is aired ordinary idle time, such as at midnight, without, utilizing an ordinary broadcasting time period.
Accordingly, it is one object of the present invention to provide an improved subscription television system in which a scrambled television signal is transmitted by way of a broadcasting wave.
It is another object of the present invention to provide a system in which a broadcast scrambled television signal is temporarily recorded, and, upon reproduction, the scrambled television signal is unscrambled for display on a conventional TV screen.
In accordance with an aspect of this invention, a subscription television system comprises means for scrambling at least one of the video signal and the audio signal of a specific program in accordance with a predetermined scrambling mode, means for generating a control signal in response to the scrambling mode, with the control signal including a key signal which is used for unscrambling the signal scrambled in the scrambling means, and means for transmitting key signal after the termination of the specific program.
The above, and other aspects, features, and advantages of the present invention will be apparent from the following detailed description of illustrative embodiments thereof which is to be read in conjunction with the accompanying drawings.